In conventional electrical machines, fans are commonly used to enhance cooling of the machine, allowing for increased power density over that which can be achieved by the machine without a cooling fan. Cooling fans can be shaft mounted or separately attached electric machine. FIG. 1 illustrates two examples of conventional cooling fans and their attachment to electric machines. Referring to FIG. 1, the lower drawing illustrates an electric machine 10 where a fan 12 is mechanically attached to a shaft 14 of machine 10. The upper drawing in FIG. 1 illustrates another electric machine 16 where a fan assembly 18 is separately attached to a non-drive end of electric machine 16. In this example, fan assembly 18 requires a separate drive machine 20 dedicated to driving the fan.
Shaft mounted fans are typically bidirectional, which allows for flexibility in application, but with drawbacks of noise and efficiency of air flow. Separately mounted fans add significant cost, mass and losses to the system. Shaft-mounted fans allow for effective cooling near the fan end of the machine. However, cooling at regions of the machine that are axially spaced from the fan is less. In addition, due to the shaft mounting, there is significant stiffness and robustness to the fan, which requires a fan guard to keep fingers and other items from becoming entangled in such fan.
In an example machine of 15 kW rating, and 4 pole design, the power required to drive a shaft-mounted fan may be on the order of 50 W, or about 0.25 Nm, while the machine has a torque of about 80 Nm. As the fan is attached to the non-drive end of the shaft, there exists a passage for the shaft and possibly required sealing, which attempts to reduce contamination of the interior elements of the machine from external sources. This particular shaft mounted fan may result in quite high convection coefficients near the fan, being around 60-100 W/m2K in some examples, which may be referred as the non-drive end (NDE) of the machine. On the end of the machine nearer the shaft, which is connected to the driven equipment, the convection coefficients may be around 20 W/m2K. Thus, the temperature of the frame surface near the drive end (DE) is substantially higher than the fan end, and the interior portions of the machine near the drive end are also at substantially higher temperatures compared to the fan end of the machine. These high temperatures have detrimental effects on the losses of the windings and the lifetime of the bearings and winding insulations, which are desired to be reduced.
In light of these difficulties, there exists a need for an improved fan assembly for an electric machine and for an electric machine that incorporates such a fan assembly.